As is well known, polyblends of rubber with monovinylidene aromatic hydrocarbons have significant advantages in providing compositions of desirable resistance to impact for many applications. Various processes have been suggested or utilized for the manufacture of such polyblends including emulsion, suspension and mass polymerization techniques, and combinations thereof. Although graft blends of a monovinylidene aromatic hydrocarbon and rubber prepared en masse exhibit desirable properties, this technique has a practical limitation upon the maximum degree of conversion of monomers to polymer which can be effected because of the high viscosities and accompanying power and equipment requirements, which are encountered when the reactions are carried beyond a fairly low degree of completion after phase inversion takes place. As a result, techniques have been adopted wherein the initial polymerization is carried out en masse to a point of conversion beyond phase inversion at which the viscosity levels are still of practical magnitudes, after which the resulting prepolymerization syrup is suspended in water or other inert liquid and polymerization of the monomers carried to substantial completion.
Polyblends having diene rubbers grafted with monovinylidene aromatic monomers can be prepared by mass-suspension polymerization methods as disclosed in U.S. Pat. No. 3,488,743 to Baer et al.
The polyblends of the present invention are known commercially as high impact polystyrene (HIPS). The HIPS polyblends are used commercially in large quantities in the packaging field for molded containers for foods. Such applications require low residual monomer contents in the finished polyblend to insure acceptance for food packaging. Prior art processes have used high temperatures and catalysts having longer half-lives at higher temperature to force the polymerization in suspension to substantial completion so as to realize residual unpolymerized monomer content in the final polyblend of about 0.5%.
Such processes have not been successful in both lowering the residual monomer content of the polyblend and preserving the efficiency of the rubber phase to maintain the toughness of the polyblend. The higher temperature polymerization and higher temperature catalysts crosslink the rubber excessively during the final conversion of the monomers causing the rubber to lower in swelling index and efficiency. In addition, the higher temperature systems lower the molecular weight of the rigid phase of the polyblend further sacrificing toughness. Such prior art processes have often resorted to post devolatilization, e.g., by extrusion, attempting to lower the residual monomer content of the polyblend. Here, however, physical degradation during extrusion lowered the polyblend properties needed.
A process has been developed to lower the residual monomer of the polyblends by using the catalyst system of the present invention as disclosed in the copending application of Norman L. Hardwicke, U.S. Ser. No. 494,647, filed Aug. 5, 1974. It has now been discovered unexpectedly that the polymerization rates of the aforementioned process can be greatly improved by using said catalyst system in combination with a saturated aliphatic acid and a compound selected from the group consisting of a hydrated salt and water or mixtures thereof providing an improved process for polymerizing polyblends.